This invention relates to spectral measuring devices, and in particular to devices of the scanning monochromator type.
A scanning monochromator consists of an entrance aperture, a rotatable dispersing optical element, which is typically a diffracting grating but sometimes a prism, a drive mechanism for rotating the dispersing optical element, an exit aperture, and any additional optical elements which collimate or focus radiation that enters via the entrance aperture. Radiation entering the monochromator through the entrance aperture and dispersed by the diffraction grating is imaged at the exit aperture as a narrow wavelength band of radiation. The bandwidth of this radiation depends upon groove density of the diffraction grating, the focal length of the optical system, the width of the entrance and exit apertures, and the incident and diffracted angles off the diffraction grating. The dispersed light which exits the monochromator system follows the well-known grating relationship, discussed at pp. 16-17 of "Diffraction Grating Handbook", Bausch & Lomb, Incorporated, Rochester, N.Y. (1970).
Historically, the diffraction grating has been mounted on a platform and rotated by a drive motor through a mechanical drive assembly that includes a sine bar and lead screw. The sine-bar drive mechanism has the advantage of producing a linear change in wavelength at the exit aperture of the monochromator, with respect to the lead screw which drives the sine bar, due to the sinusoidal relationship between grating rotation and dispersion. This has facilitated the use of a linear, mechanical counter attached to the shaft of the lead screw, for display of the wavelength of radiation exiting the exit aperture. But this arrangement has several limitations. First, the lead screw is subject to dust or dirt contamination which can dramatically affect accuracy and performance of the drive system. Second, the lead screw must be lubricated with oil, requiring frequent oiling maintenance to insure proper lubrication. Third, the sine bar rides on a carriage attached to the lead screw; this typically employs a ball and a flat which rub against each other. This is a source of mechanical wear and, if wear is severe enough, the wear is a source of inaccuracy and imprecision of wavelength registration. Fourth, although the accuracy and precision of the drive system can be improved by increasing the length of the sine bar and lead screw, the longer and more costly these devices become, and the more time it takes to rotate the diffraction grating through a given angle. Additionally, the speed at which the lead screw can be rotated is limited by the speed at which oil is literally slung off the lead screw.
With the widespread use of personal computers and microprocessors to control the movement of scanning monochromators, it is no longer essential to have a mechanical wavelength counter attached to the grating drive mechanism. The wavelength can be calculated or derived from a "look-up" table if the drive mechanism does not have a sinusoidal response, as does the sine bar/lead-screw mechanism. This circumstance has facilitated use of drive mechanisms that directly rotate the dispersing element of the monochromator. Examples of direct-drive mechanisms are the galvanometer drive, as in Bernier et al. U.S. Pat. No. 4,469,441, and the worm-gear drive.
The galvanometer-drive system utilizes a galvanometer, coupled directly to the diffraction grating, to rotate the dispersing element. The drive is simple and fast, yet it can suffer from hysteresis when it stops at a particular position, and it must utilize a very high resolution input signal (usually a digital-to-analog converter of 16 or more bits, i.e., 2.sup.16 or more) to control the current to the galvanometer, since current to the galvanometer is related to grating rotation. More importantly, however, the drive has a limited usable range of angular rotation, thereby limiting the wavelength coverage of the monochromator. For example, for angular deflections of about 15.degree., the galvanometer produces enough heat to change its resistance (impedance), and the change in impedance changes the amount of current needed to rotate the drive. Thus, as the temperature changes, so does the calibration of the drive, in that the needed current per degree of rotation is changing, and it is difficult to reproduce a wavelength (at the exit aperture) or angular position (of the dispersing element) with any precision or accuracy.
In the worm-gear arrangement, the diffraction grating is mounted to a rotatable platform, motor-driven via a worm-gear transmission. The worm-gear transmission is susceptible to gear wear as a function of time, leading to poor accuracy and precision, and ultimately requiring replacement of the transmission.